1. Field of the Invention
The present invention relates to solar cells, and more particularly, to solar cells having a semicystalline structure. An inexpensive solar cell is provided which can be constructed in large sections or grains and has a conductor network specifically located over the grain boundaries of the semicrystalline material.
2. Description of the Prior Art
Solar cells are generally constructed of a semiconductor material which converts incident radiation such as sunlight to electricity. This process is often referred to as photovoltaic conversion. The semiconductor material is often a wafer of "single crystal" silicon which has a single crystalline structure of a particular orientation throughout the wafer. The wafer is doped with impurities to form a p-n junction. A metal grid is usually deposited on the surface of the wafer to collect the electricity that is generated. The metal grid typically covers from 8 to 11% of the surface depending upon the size of the wafer. Since the entire wafer area of a single crystal solar cell is active, that is, participates in the photovoltaic conversion, the metal grid covering the surface shields active area from the incident radiation. This contributes to lowering the achievable efficiency of the solar cell.
Furthermore, single crystal silicon is relatively expensive and is limited in shape and size. Wafers of single crystal silicon are sliced from round ingots which generally do not exceed 10 centimeters in diameter. A less expensive material is semicrystalline silicon which can be cast in much larger rectangular ingots. A semicrystalline wafer has several grain areas each of which has a particular crystalline orientation or grain which usually differs from neighboring grain areas. The crystal grain areas are separated by grain boundaries which are discontinuities in the silicon crystalline structure. The term semicrystalline is meant here to encompass what is also commonly called polycrystalline.
Although less expensive than single crystal solar cells, prior attempts to construct cells from semicrystalline material have experienced several difficulties. First, the grain boundaries are inactive and therefore do not contribute to the photovoltaic generation of electricity. Thus, a semicrystalline solar cell has less active area than a single crystal cell of equivalent size.
In addition, when the semicrystalline wafer is doped with impurities to form the p-n junction, the grain boundaries have a tendency to become doped to a deeper level and to a greater degree than the grain areas. This usually causes shorting or leakage to occur across the p-n junction. Also, because of the discontinuous structure of the grain boundary, the carriers generated by the cell can recombine more easily in the grain boundaries so that the grain boundaries can act as a current sink. Furthermore, it has been found that incident sunlight on the grain boundaries further increases the recombination rate in the grain boundary.
Attempts have been made to passivate the grain boundary by the use of monoatomic hydrogen or other techniques to prevent the grain boundary from acting as a current sink or short. However, these studies have not provided a highly practical process or processes which can be used to produce solar cells using semicrystalline material in large volumes in a manufacturing environment utilizing state of the art technology.